After today, it will be open for signing until April 21, 2017. And even after that, additional states can still decide to adopt the agreement through accession, which is more or less United Nations code for "coming late to the party."

Think tank Third World Network has also urged developing nations to hold off joining until they see concrete plans from wealthy nations, pointing to a history of empty promises to combat climate change.

Signatories have pledged to create climate plans aiming to limit global warming to 2.7 degrees Fahrenheit, with the more realistic (but perhaps arbitrary) goal of keeping global temperatures no more than 3.6 degrees above pre-industrial levels. (The pact is big on goals, but short on the details of how to achieve them.)

These past failures should guide world leaders as they pursue the Paris Agreement, which like its predecessors is not legally binding— yet.

President Obama meets with Indian Prime Minister Narendra Modi at the climate change summit in Paris.Kevin Lamarque/ReutersThe multi-step process is long and complicated, and there are lots of junctures where it could be derailed.

Once 55 countries, whose collective emissions must add up to at least 55% of the global total, have reached that step, the accord becomes legally binding and members states will be expected to fulfill their commitments.

The US and China, both of which have announced their intent to become official member states, account for 40% of global emissions. Both will likely ratify their country's participation through executive action.

And then there's the US presidential election. While President Barack Obama has pledged to follow through on US commitments, the next president could, in theory, pull out of the deal, effectively halting progress on reducing emissions.

While far from perfect, the Paris Agreement remains a promising development for world action on climate change, and the April 22 signing ceremony has world leaders promising to pursue climate-change solutions at home and abroad.

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